First off, since you haven't defined what you deem as "the best", you can receive biased opinion of people who have used modal dialogs.
I've used quite a few, and I'll say that hands down - jQuery dialog is the best in my opinion.
jQuery isn't that big and you can load it into your page from google pages.
The other thing is that no matter what modal window you want to use, you need to use some logic the author provides which can fit to the category of learning new language, even tho it's only a few lines of code.
The other thing is, if you have ever used css - it will take MINUTES to get acquainted with jQuery. If your reason for avoiding jQuery is that you don't want to learn a new language - why even bother with web development then?
It takes less time to set up a modal window with jQuery UI than to post this topic and ask for opinion.